Many methods are known for forming the electrical interconnections between an integrated circuit and the supporting substrate. Tape automated bonding (TAB) is one commonly known method for forming these such electrical interconnections. A TAB tape is provided which comprises a plurality of individual long, slender inner leads attached to, and extending out from, generally wider, larger outer leads. There may be many of these inner/outer lead configurations on a single TAB tape.
An individual inner lead on the TAB tape is bonded to the integrated circuit at a bonding pad so as to form in inner lead bond. There are typically many of these inner lead bonds on a single integrated circuit. The inner lead bonds are typically formed by first depositing a gold bump, or other suitable material, on either the end of the TAB tape inner lead or on the integrated circuit itself. The integrated circuit and TAB tape inner leads, which are generally copper, are then aligned and simultaneously thermocompression gang bonded.
After bonding between the integrated circuit and inner leads is complete, the integrated circuit is excised from the TAB tape at a point beyond the outer lead, so that the outer lead remains attached to the bonded inner lead and integrated circuit. The integrated circuit assembly is subsequently mounted on a substrate, if this has not already been done, and the outer leads are appropriately bonded to the substrate.
A significant drawback associated with the tape automated bonding method for forming the electrical interconnections between the integrated circuits and substrates is that the individual inner leads are highly susceptible to deformation. The individual inner leads are both thin and narrow, extending out from the more structurally durable outer leads in an unsupported manner. Therefore, the inner leads are extremely fragile and highly susceptible to deformation during processing and handling. This high susceptibility to deformation renders processing of the inner leads very difficult without disturbing the inner leads' position and location relative to the integrated circuit, substrate and other inner leads.
The use of these fragile inner leads results in higher rejection rates and lower levels of reproducibility than desired. In addition, the use of individual thin and narrow inner leads effects the electrical performance of the integrated circuit by limiting the current carrying capacity, increasing the electrical resistance, and limiting the heat transfer from the integrated circuit to the substrate through the interconnection.
In an alternative bonding method, a flexible circuit (FLEX) is used to form the electrical interconnections between the integrated circuit and the substrate, the substrate being an integral part of the flexible circuit itself. The FLEX circuit consists of a patterned arrangement of conductors on a flexible insulating base substrate with or without cover layers. The FLEX circuit may be single or double sided, multi-layered, or rigidized, in addition to other possible arrangements. The FLEX circuit may be formed by several methods, such as by laminating copper foil to any of several base substrate materials, or alternatively pattern plating copper directly onto the substrate.
The FLEX circuit is advantageous in that it contains both the internal and external integrated circuit chip interconnections. The inner leads are adjacent to and an integral part of the flexible circuitry pattern. Outer leads are not required because the individual inner leads are incorporated within the flexible circuitry pattern. In addition, the flexible circuitry pattern is supported by the flexible insulating substrate and electrically connected at the appropriate regions. Therefore outer lead bonds are not necessary and correspondingly the number of interconnections are substantially reduced.
For these reasons, flexible circuitry technology has many advantages. FLEX circuitry significantly reduces the number of chip interconnections resulting in reduced lead inductance and lead-to-lead capacitance, as well as increased product reliability. In addition, the use of the FLEX circuitry permits smaller integrated circuits and interconnection patterns because the chip is mounted directly onto the patterned substrate.
However, despite these advantages of FLEX circuitry, a significant disadvantage exists --similar to the disadvantage encountered with the use of TAB tapes. The inner leads extending from the flexible circuitry pattern are individual fragile strips of copper. Analagous to the TAB inner leads, these leads are narrow strips extending beyond the main, more structurally durable, circuit pattern supported by the flexible substrate. As with the TAB inner leads, this fragile configuration is highly susceptible to deformation and results in the inner leads being extremely difficult to process or handle without disturbing the lead's position or location relative to the FLEX circuitry substrate, the integrated circuit and the other inner leads. This produces higher than desired rejection rates and decreased electrical properties, similar to the problems encountered with tape automated bonding.
It is therefore advantageous to provide an inner interconnection lead, suitable for bonding to an integrated circuit for electrically interconnecting the integrated circuit to a substrate, which is both structurally durable and electrically efficient. It is further desirable that the interconnection lead have a low susceptibility to deformation. In addition, it is desirable that the provided interconnection lead be suitable for use with either tape automated bonding or flexible circuitry technologies.